1. Field of the Invention
The present invention relates to an exhaust gas cleaning system of an internal combustion engine having a particulate filter in an exhaust passage. Specifically, the present invention relates to an exhaust gas cleaning system capable of performing temperature increase regeneration of a particulate filter.
2. Description of Related Art
In recent years, an exhaust gas cleaning system, which reduces discharge of toxic components by treating exhaust gas discharged from an internal combustion engine with a catalyst or a filter, has gained importance as a measure to conserve the environment. For instance, an exhaust gas cleaning system having a diesel particulate filter (DPF) disposed in an exhaust pipe for collecting particulate matters discharged from the diesel engine is known. The DPF is regenerated by combusting and eliminating the accumulated particulate matters regularly. Thus, the DPF can be used continuously.
The regeneration of the DPF is performed by increasing the temperature of the DPF to certain temperature (for instance, 600° C. or above), at which the particulate matters can be combusted, when the quantity of the accumulated particulate matters (a particulate matter accumulation quantity PM, hereafter) reaches a predetermined value. The particulate matter accumulation quantity PM is calculated based on a pressure difference across the DPF. At that time, temperature increasing means performs post-injection, retardation of fuel injection timing, restriction of intake air or the like. However, such temperature increasing methods deteriorate fuel consumption.
As the temperature T of the DPF for performing the regeneration is increased, combustion velocity of the particulate matters is increased and the regeneration is finished in a short time length. As a result, the deterioration in the fuel consumption accompanying the regeneration of the DPF can be reduced. However, if the DPF temperature T is too high, there is a possibility of damage to the DPF, degradation of an oxidation catalyst supported by the DPF or the like as shown in FIG. 13. In FIG. 13, a solid line “v” represents the combustion velocity of the accumulated particulate matters, another solid line “f” is the degree of the deterioration in the fuel consumption and an area H is a temperature area where there is a possibility of the degradation of the oxidation catalyst or the damage to the DPF. Therefore, in order to inhibit the deterioration in the fuel consumption and to perform the regeneration of the DPF safely, the DPF temperature T has to be maintained near predetermined temperature. Therefore, usually, temperature of the exhaust gas upstream or downstream of the DPF is sensed and the temperature increasing means is operated so that the sensed temperature coincides with the target temperature.
In a technology disclosed in Japanese Patent Application Unexamined Publication No. H11-101122, an oxidation catalyst (a diesel oxidation catalyst: a DOC, hereafter) is disposed upstream of the DPF in series as shown in FIG. 14A, and the temperature of the exhaust gas upstream of the DPF and downstream of the DOC is sensed as the DPF temperature T. Then, as shown in FIG. 15, if the DPF temperature T exceeds a predetermined value (for instance, 500° C.), the temperature increasing operation by the temperature increasing means is stopped (as shown by a state “OFF” in a solid line “T-UP” in FIG. 15). If the DPF temperature T becomes lower than the predetermined temperature (for instance, 500° C.), the temperature increasing operation is performed by the temperature increasing means (as shown by a state “ON” in the solid line T-UP in FIG. 15). In FIG. 15, an area L represents a temperature area where the accumulated particulate matters cannot be combusted.
However, the above technology only performs the operation for switching the temperature increasing means, which performs the post-injection, for instance, between an operated state and a stopped state. Therefore, if the post-injection is stopped (OFF) at a time point tA in FIG. 16 when the temperature of the exhaust gas shown by a thin line “b” in FIG. 16 approaches the predetermined target temperature Tt during the operation for switching between performance and interruption of the post-injection, the temperature of the DOC decreases rapidly as shown by a broken line “a” in FIG. 16. It is because low-temperature exhaust gas enters the DOC and generation of reaction heat of hydrocarbon stops. The change of the sensed temperature of the exhaust gas upstream of the DPF shown by the thin line “b” in FIG. 16 is delayed with respect to the change in the DOC temperature shown by the broken line “a”. Therefore, at this time point, the sensed temperature shown by the thin line “b” is maintained at high temperature for a while. A graph shown in FIG. 14B shows temperature distribution in the exhaust pipe shown in FIG. 14A at that time, based on temperatures sensed at points P1–P7 shown in FIG. 14A.
More specifically, the post-injection is interrupted (OFF) while the DPF upstream exhaust gas temperature shown by the thin line “b” is maintained at the high temperature. The post-injection is restarted when the DPF upstream exhaust gas temperature shown by the thin line “b” becomes lower than the target temperature (for instance, 500° C.) Tt at a time point tB in FIG. 16. The DOC temperature has been decreased largely as shown by the broken line “a” in FIG. 16 by the time when the post-injection is restarted at the time point tB. Therefore, the low-temperature exhaust gas passing through the low-temperature DOC enters the DPF. As a result, the DPF temperature downstream of the DOC decreases largely once as shown by a heavy line “c” in FIG. 16 in spite of the fact that the post-injection is performed.
Therefore, it takes a long time before the DOC temperature is increased by the hydrocarbon reaction heat generated through the restart of the post-injection as shown by the broken line “a”, and subsequently the temperature of the DPF downstream of the DOC recovers to the proximity of the target temperature Tt as shown by the heavy line “c”. When the post-injection is performed but the DPF temperature is low (for instance, 450° C. or lower), the combustion velocity of the particulate matters on the DPF is low. In the state, the fuel consumption is deteriorated because of the post-injection, but little or no particulate matters on the DPF can be combusted.